1. Technical Field of the Invention
The present invention relates to a rotating/rotatable member such as a blade or labyrinth seal for use in a gas turbine, steam turbine, compressor or the like, and a method for coating the rotating/rotatable member. More particularly, it relates to a rotating/rotatable member on a part of which a coating film including a hard material is formed, and a method for coating the rotating/rotatable member.
2. Description of the Related Art
For a rotating/rotatable member such as a blade or a labyrinth seal, a clearance between a rotating section and a stationary section, such as a chip clearance between the blade and a casing or a shroud, or a seal clearance between the labyrinth seal and a honeycomb seal, needs to be kept/set to be appropriate during operation of a gas turbine. When the clearance is set to be excessively large to avoid contact, efficiency of the gas turbine drops. Conversely, when the clearance is set to be excessively small, a tip end of the rotating member breaks and causes trouble for the gas turbine.
Therefore, in consideration of contact of the rotating member with surrounding members (casing, shroud, honeycomb seal, and the like), a tip end of a blade or of a labyrinth seal is coated with an abrasive coating of a relatively hard material for chipping off the material of a contact surface of the surrounding member. The surrounding member is coated with an abradable coating of a material, which is relatively easily chipped. Accordingly, chip clearance or seal clearance is adjusted so as to be minimized so that a side of the surrounding member is chipped off by the tip end of the rotating member when driving the gas turbine thereby taking advantage of a hardness difference of the coatings.
In this case, FIG. 1A is a perspective view of a usual turbine blade, FIG. 1B is a perspective view of the turbine blade with a chip shroud, and FIG. 1C is a perspective view of a compressor blade. It is to be noted that a platform or a dovetail on a turbine disk side is omitted from these figures. In a turbine blade 1, shown in FIG. 1A, the whole surface of a blade tip end is coated with an abrasive coating 5a. In a turbine blade 2 provided with a chip shroud 3, shown in FIG. 1B, the whole surfaces of the tip ends of chip fins 4 disposed on a chip shroud 3 (i.e., the tip ends of the turbine blade) are coated with abrasive coatings 5b. Furthermore, for the blade 1 of the compressor, shown in FIG. 1C, an abrasive coating 5c is applied over the region of the blade tip end (including the backside of the figure).
Moreover, FIG. 2 is a sectional view showing one example of a labyrinth seal tip end. The labyrinth seal is disposed in the clearance between a rotating section and a stationary section to prevent leakage of air or combustion gas, and is a seal structure frequently used in a gas turbine and compressor. In general, an annular labyrinth seal 6 including concave/convex portion is disposed on a rotating section side, and a honeycomb seal (not shown) including a structure easy to be chipped off is disposed on a stationary section side. FIG. 2 illustrates a sectional view cut in a plane including a center axis of the labyrinth seal 6, and an abrasive coating 5d is applied to the tip end of the convex portion of the labyrinth seal 6.
These abrasive coatings have heretofore been applied by methods such as welding, thermal spraying, and plating (e.g., see References 1 and 2). With respect to coating by welding, a welding rod or a powder body is used to coat predetermined portions, such as the tip end of the turbine blade or the labyrinth seal. With respect to coating by thermal spraying, zirconia is thermally sprayed, which has a small difference in thermal expansion from a mother material and whose hardness is relatively high (Vickers hardness of 1300 HV). With respect to coating by plating, abrasive grains (Vickers hardness of 4500 HV) of cubic boron nitride (cBN), which are high in hardness, are electrically attached by nickel plating.
It is to be noted that other prior art methods related to the present invention are described in References 3, 4.
[Reference 1]
Japanese Laid-Open Patent Publication No. 11-286768.
[Reference 2]
Japanese Laid-Open Patent Publication No. 2000-345809.
[Reference 3]
Japanese Laid-Open Patent Publication No. 7-301103.
[Reference 4]
Japanese Laid-Open Patent Publication No. 8-319804.
However, in the above-described methods, a portion that does not have to be coated is masked in order to closely attach the abrasive coating, and the surface to be coated needs to be blast-treated in order to enhance adhesion, and there are problems in that there many pretreatments are required and costs are high. In either conventional thermal spraying or plating methods, there have been problems in that the adhesion of the coating is bad, peeling occurs at the time of driving the apparatus, engine trouble is caused, and additionally the chip clearance or the seal clearance is not maintained appropriately. Furthermore, there is a problem in that, with respect to coating by welding, only a metal much lower in hardness can be coated as compared to when a ceramic is used, and, therefore, abrasive properties (which are properties for chipping off a material to be ground) are inferior. Moreover, there is a problem in that the quality level of the coating fluctuates based on the operator's expertise, and a welding crack may easily occur when employing a material poor in thermal conductivity and having small elongation properties. Furthermore, there has been a problem encountered in that post-treatments are required, such as processing grinding to a required dimension after welding so that a lot of trouble is required.
Moreover, according to References 3 and 4, in the coating method, discharge is performed between the rotating member and an electrode on first discharge conditions so that the electrode is consumed, and the electrode is formed in accordance with the shape of a coating film forming portion. Thereafter, the coating film is formed by discharge between the electrode and the rotating member on second discharge conditions. Then, even when the electrode is not processed beforehand for a product shape, a coating object portion can still be appropriately coated. On the first discharge conditions for consuming the electrode, the electrode is set to have a minus polarity, a pulse width is set to 1 μs or less, and a current value is set to 10 A or less. On the second discharge conditions for forming the coating film, the electrode is preferably set to have minus polarity, the pulse width is set to be 2 to 10 μs, and the current value is set to be 5 to 20 A.
Moreover, in accordance with conventional abrasive coating, because the whole area of the tip end of the blade is coated, there has been a problem encountered in that the coating range is broad and the yield of products is poor.
Furthermore, heretofore, coating has been performed by plating or by thermal spraying. Therefore, during production (manufacturing) of the labyrinth seal, coating pretreatments, such as a blast process and a process of attaching a masking tape, are required before coating is performed, and coating post-treatments, such as a process of removing the masking tape, are required after coating is performed. Therefore, the operation time required for the production (manufacturing) of the labyrinth seal lengthens, and, therefore, it is not easy to improve productivity of the labyrinth seal.
Additionally, for the same reason, the abrasive coat cannot be firmly attached to the tip edge of a seal fin. Therefore, a problem has been encountered in that the abrasive coat easily peels off the tip edge of the seal fin and the quality of the labyrinth seal is not stable.